The Chlamydiae are obligate intracellular parasites of eukaryotic cells which are responsible for endemic sexually transmitted infections and various other disease syndromes. They occupy an exclusive eubacterial phylogenic branch, having no close relationship to any other known organisms. A particular characteristic of the Chlamydiae is their unique life cycle, in which the bacterium alternates between two morphologically distinct forms: an extracellular infective form (elementary bodies, EB) and an intracellular non-infective form (reticulate bodies, RB). The life cycle is completed with the re-organization of RB into EB, which leave the disrupted host cell ready to infect further cells.
The genome sequences of at least five Chlamydia trachomatis or chlamydophila species are currently known—C. trachomatis, C. pneumoniae, C. muridarum, C. pecorum and C. psittaci [1, 7]. The various C. trachomatis strains, of which there are currently at least 18 serovariants (serovars), may be classified according to their serological reactivities with polyclonal or monoclonal antisera. These serological differences are typically detected due to differences in the MOMP (Major Outer Membrane Protein) of C. trachomatis. 
Although Chlamydia infection itself causes disease, it is thought that the severity of symptoms in some patients is actually due to an aberrant or an altered host immune response which may arise from either (i) the nature of the invading Chlamydia organism which may vary from serovar to serovar or (ii) the nature of the subject invaded (for example, the nature of the patient profile). The failure to clear the infection results in persistent immune stimulation and, rather than helping the host, this results in chronic infection with severe consequences, including sterility and blindness [8]. In addition, the protection conferred by natural Chlamydial infection is usually incomplete, transient, and strain-specific.
Unfortunately the major determinants of Chlamydia pathogenesis are complicated and at present still unclear, mostly due to the intrinsic difficulty in working with this pathogen and the lack of adequate methods for its genetic manipulation. In particular very little is known about the antigenic composition of elementary body surface, that is an essential compartment in pathogen-host interactions, and likely to carry antigens able to elicit a protective immune response.
Due to the serious nature of the disease, there is a desire to provide suitable immunogenic compositions, such as vaccines to deal with an aberrant or altered host cell immune response which may result from, for example, allelic variation in the invading Chlamydia strain and/or aberrant or altered forms of Chlamydia invading strain. These immunogenic compositions may be useful (a) for immunisation against Chlamydial infection or against Chlamydia-induced disease (prophylactic vaccination) or (b) for the eradication of an established chronic Chlamydia infection (therapeutic vaccination). Being an intracellular parasite, however, the bacterium can generally evade antibody-mediated immune responses.
Various antigenic proteins have been described for C. trachomatis, and the cell surface in particular has been the target of detailed research [9]. These include, for instance, Pgp3 [10, 11, and 12], MOMP [13], Hsp60 (GroEL) [14] and Hsp70 (DnaK-like) [15]. Not all of these have proved to be effective vaccines, however, and further candidates have been identified [16]. Compositions comprising combinations of C. trachomatis antigens are described in reference 17.
Vaccines against pathogens such as hepatitis B virus, diphtheria and tetanus typically contain a single protein antigen (e.g. the HBV surface antigen, or a tetanus toxoid). In contrast, acellular whooping cough vaccines typically have at least three B. pertussis proteins, and the Prevnar™ pneumococcal vaccine contains seven separate conjugated saccharide antigens. Other vaccines such as cellular pertussis vaccines, the measles vaccine, the inactivated polio vaccine (IPV) and meningococcal OMV vaccines are by their very nature complex mixtures of a large number of antigens. Whether protection can be elicited by a single antigen, a small number of defined antigens, or a complex mixture of undefined antigens, therefore depends on the pathogen in question.
It is an object of the invention to provide further and improved immunogenic compositions for providing immunity against Chlamydial disease and/or infection. In particular, it is an object of the invention to provide improved immunogenic compositions for providing immunity against aberrant or altered Chlamydia serovar strains (e.g. strains such as allelic variant strains).
It is an object of the invention to provide further and improved compositions for providing immunity against chlamydial disease and/or infection. The compositions are based on a newly discovered, surface-exposed C. trachomatis antigens.